Project Summary Over 5.7 million Americans are afflicted with Alzheimer's disease (AD), and 2/3 of them are women. Indeed, female sex is a leading risk factor for AD (second only to aging itself), which is correlated with the transition to the postmenopausal state. A further exacerbation in the female aging process is the increased incidence of obesity and type 2 diabetes, which are also major risk factors for AD in both women and men. Therefore, a combination of ovarian steroid dysfunction and metabolic disturbances may play a significant role in provoking AD in women. The goals of our funded research are to elucidate the mechanism(s) by which metabolic states and 17?-estradiol (E2) regulate arcuate nucleus kisspeptin (Kiss1ARH) neuronal circuits that are critical for coordinating energy homeostasis and reproduction in females. It is well known that E2 is anorexigenic, and Kiss1 neurons, which are directly regulated by E2, are essential for reproductive success. In addition, we have documented that glutamate is released from Kiss1ARH neurons and excites the anorexigenic proopiomelanocortin (POMC) neurons while inhibiting the orexigenic neuropeptide Y/agouti-related peptide (AgRP) neurons by activating separate groups of metabotropic glutamate receptors. Thus, there is compelling evidence that Kiss1ARH neurons integrate metabolic and steroid hormonal cues to regulate energy homeostasis via glutamatergic transmission. Recently, using ovariectomized (OVX) animals as a model of a hypoestrogenic state (as is seen in aging), we have elucidated the neuroprotective effects of E2 within this hypothalamic circuitry. Moreover, recent clinical evidence has correlated disturbances in this neuronal circuitry with AD, but essentially nothing is known about the underlying mechanisms. Therefore, we predict that the reduction in excitability and glutamatergic synaptic transmission by Kiss1ARH neurons will be exacerbated following ovariectomy in 5XFAD mice, leading to obesity and accelerated neuropathology in this model of early onset AD. Our multidisciplinary approach combines cellular, molecular, electrophysiological, and optogenetic tools with whole animal physiology to address the following aims: 1) To elucidate the direct synaptic input to POMC and AgRP neurons from Kiss1ARH neurons using optogenetic tools in combination with whole-cell recording in vehicle- and E2-treated OVX 5XFAD females at different time-points in the disease progression; and 2) to elucidate in Kiss1ARH neurons the expression and function of T-type calcium channels (Cav3) and hyperpolarization-activated, cyclic nucleotide-gated (Hcn) ion channels and the expression of Vglut2 mRNA in adult 5XFAD animals following OVX and E2-replacement. Elucidating the signaling cascades underlying the actions of E2 in the hypothalamus in healthy versus AD model animals will provide a framework for understanding the mechanism by which metabolic dysfunction contributes to the progression of AD.